Publications by authors named "Clemens von Korff Schmising"

13 Publications

  • Page 1 of 1

Toward ultrafast magnetic depth profiling using time-resolved x-ray resonant magnetic reflectivity.

Struct Dyn 2021 May 23;8(3):034305. Epub 2021 Jun 23.

Sorbonne Université, CNRS, Laboratoire de Chimie Physique-Matière et Rayonnement, 75005 Paris, France.

During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump-probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm ( ), we were able to probe close to the Fe edge ( ) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.
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http://dx.doi.org/10.1063/4.0000109DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8225393PMC
May 2021

Observation of fluctuation-mediated picosecond nucleation of a topological phase.

Nat Mater 2021 Jan 5;20(1):30-37. Epub 2020 Oct 5.

European XFEL, Schenefeld, Germany.

Topological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states.
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http://dx.doi.org/10.1038/s41563-020-00807-1DOI Listing
January 2021

Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges.

Phys Rev Lett 2020 Sep;125(12):127201

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2A, 12489 Berlin, Germany.

We systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M_{3,2} absorption resonances using the [email protected] free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16  mJ/cm^{2}/pulse to 10 000  mJ/cm^{2}/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulse-implying an increased XUV peak electric field-results in a reduced quenching of the resonant diffraction at the Co M_{3,2} edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples.
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http://dx.doi.org/10.1103/PhysRevLett.125.127201DOI Listing
September 2020

A tabletop setup for ultrafast helicity-dependent and element-specific absorption spectroscopy and scattering in the extreme ultraviolet spectral range.

Rev Sci Instrum 2020 Sep;91(9):093001

Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße 2A, 12489 Berlin, Germany.

Further advances in the field of ultrafast magnetization dynamics require experimental tools to measure the spin and electron dynamics with element-specificity and femtosecond temporal resolution. We present a new laboratory setup for two complementary experiments with light in the extreme ultraviolet (XUV) spectral range. One experiment is designed for polarization-dependent transient spectroscopy, particularly for simultaneous measurements of magnetic circular dichroism (MCD) at the 3p resonances of the 3d transition metals Fe, Co, and Ni. The second instrument is designed for resonant small-angle scattering experiments with monochromatic light allowing us to monitor spin dynamics with spatial information on the nanometer scale. We combine a high harmonic generation (HHG) source with a phase shifter to obtain XUV pulses with variable polarization and a flux of about (3 ± 1) × 10 photons/s/harmonic at 60 eV at the source. A dedicated reference spectrometer effectively reduces the intensity fluctuations of the HHG spectrum to below 0.12% rms. We demonstrate the capabilities of the setup by capturing the energy- and polarization-dependent absorption of a thin Co film as well as the time-resolved small-angle scattering in a magnetic-domain network of a Co/Pt multilayer. The new laboratory setup allows systematic studies of optically induced spin and electron dynamics with element-specificity, particularly with MCD as the contrast mechanism with femtosecond temporal resolution and an unprecedented signal-to-noise ratio.
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http://dx.doi.org/10.1063/5.0013928DOI Listing
September 2020

Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy.

Nat Commun 2020 Feb 13;11(1):871. Epub 2020 Feb 13.

Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Strasse 2A, 12489, Berlin, Germany.

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.
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http://dx.doi.org/10.1038/s41467-020-14691-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7018696PMC
February 2020

[email protected]: an end-station for imaging, electron- and ion-spectroscopy, and pump-probe experiments at the FLASH free-electron laser.

J Synchrotron Radiat 2018 Sep 2;25(Pt 5):1529-1540. Epub 2018 Aug 2.

PNSensor GmbH, Munich, Germany.

The non-monochromatic beamline BL1 at the FLASH free-electron laser facility at DESY was upgraded with new transport and focusing optics, and a new permanent end-station, CAMP, was installed. This multi-purpose instrument is optimized for electron- and ion-spectroscopy, imaging and pump-probe experiments at free-electron lasers. It can be equipped with various electron- and ion-spectrometers, along with large-area single-photon-counting pnCCD X-ray detectors, thus enabling a wide range of experiments from atomic, molecular, and cluster physics to material and energy science, chemistry and biology. Here, an overview of the layout, the beam transport and focusing capabilities, and the experimental possibilities of this new end-station are presented, as well as results from its commissioning.
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http://dx.doi.org/10.1107/S1600577518008585DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6140390PMC
September 2018

Generating circularly polarized radiation in the extreme ultraviolet spectral range at the free-electron laser FLASH.

Rev Sci Instrum 2017 May;88(5):053903

Max-Born-Institut Berlin, Max-Born-Str. 2a, 12489 Berlin, Germany.

A new device for polarization control at the free electron laser facility FLASH1 at DESY has been commissioned for user operation. The polarizer is based on phase retardation upon reflection off metallic mirrors. Its performance is characterized in three independent measurements and confirms the theoretical predictions of efficient and broadband generation of circularly polarized radiation in the extreme ultraviolet spectral range from 35 eV to 90 eV. The degree of circular polarization reaches up to 90% while maintaining high total transmission values exceeding 30%. The simple design of the device allows straightforward alignment for user operation and rapid switching between left and right circularly polarized radiation.
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http://dx.doi.org/10.1063/1.4983056DOI Listing
May 2017

Multi-color imaging of magnetic Co/Pt heterostructures.

Struct Dyn 2017 Jan 16;4(1):014301. Epub 2017 Feb 16.

Elettra-Sincrotrone Trieste , 34149 Basovizza, Trieste, Italy.

We present an element specific and spatially resolved view of magnetic domains in Co/Pt heterostructures in the extreme ultraviolet spectral range. Resonant small-angle scattering and coherent imaging with Fourier-transform holography reveal nanoscale magnetic domain networks via magnetic dichroism of Co at the M edges as well as via strong dichroic signals at the O and N edges of Pt. We demonstrate for the first time simultaneous, two-color coherent imaging at a free-electron laser facility paving the way for a direct real space access to ultrafast magnetization dynamics in complex multicomponent material systems.
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http://dx.doi.org/10.1063/1.4976004DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315665PMC
January 2017

Curved gratings as an integrated photon fluence monitor in x-ray transmission scattering experiments.

Opt Express 2016 Jun;24(12):13091-100

A concept to obtain a measure of the photon flux accepted by a solid sample in single-shot transmission experiments with extreme ultraviolet (XUV) or soft x-ray radiation is demonstrated. Shallow, continuously distorted gratings are used to diffract a constant fraction of the incident photons onto an extended area of a CCD detector. The signal can be tailored to fit the dynamic range of the detector, i.e. matching the scattered intensity of the studied structure of interest. Furthermore, composite grating designs that also allow for the measurement of the spatial photon distribution on the sample are demonstrated. The gratings are directly fabricated by focused ion-beam (FIB) lithography into a Si3N4 membrane that supports the actual sample layer. This allows for rapid fabrication of hundreds of samples, making the concept suitable for systematic studies in destructive single-shot measurements at free-electron laser (FEL) sources. We demonstrate relative photon flux measurements in magnetic scattering experiments with synchrotron and FEL radiation at 59.6 eV photon energy.
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http://dx.doi.org/10.1364/OE.24.013091DOI Listing
June 2016

Indirect excitation of ultrafast demagnetization.

Sci Rep 2016 Jan 6;6:18970. Epub 2016 Jan 6.

Sorbone Universités, UPMC Univ Paris 06, UMR 7614, LCPMR, 75005 Paris, France.

Does the excitation of ultrafast magnetization require direct interaction between the photons of the optical pump pulse and the magnetic layer? Here, we demonstrate unambiguously that this is not the case. For this we have studied the magnetization dynamics of a ferromagnetic cobalt/palladium multilayer capped by an IR-opaque aluminum layer. Upon excitation with an intense femtosecond-short IR laser pulse, the film exhibits the classical ultrafast demagnetization phenomenon although only a negligible number of IR photons penetrate the aluminum layer. In comparison with an uncapped cobalt/palladium reference film, the initial demagnetization of the capped film occurs with a delayed onset and at a slower rate. Both observations are qualitatively in line with energy transport from the aluminum layer into the underlying magnetic film by the excited, hot electrons of the aluminum film. Our data thus confirm recent theoretical predictions.
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http://dx.doi.org/10.1038/srep18970DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4702181PMC
January 2016

Subpicosecond hard x-ray streak camera using single-photon counting.

Opt Lett 2010 Oct;35(19):3219-21

Atomic Physics Division, Department of Physics, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden.

We have developed and characterized a hard x-ray accumulating streak camera that achieves subpicosecond time resolution by using single-photon counting. A high repetition rate of 2 kHz was achieved by use of a readout camera with built-in image processing capabilities. The effects of sweep jitter were removed by using a UV timing reference. The use of single-photon counting allows the camera to reach a high quantum efficiency by not limiting the divergence of the photoelectrons.
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http://dx.doi.org/10.1364/OL.35.003219DOI Listing
October 2010

Femtosecond holography in lithium niobate crystals.

Opt Lett 2005 Sep;30(17):2233-5

Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, USA.

Spatial gratings are recorded holographically by two femtosecond pump pulses at 388 nm in lithium niobate (LiNbO3) crystals and read out by a Bragg-matched, temporally delayed probe pulse at 776 nm. We claim, to our knowledge, the first holographic pump-probe experiments with subpicosecond temporal resolution for LiNbO3. An instantaneous grating that is due mostly to the Kerr effect as well as a long-lasting grating that results mainly from the absorption caused by photoexcited carriers was observed. The Kerr coefficient of LiNbO3 for our experimental conditions, i.e., pumped and probed at different wavelengths, was approximately 1.0 x 10(-5) cm2/GW.
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http://dx.doi.org/10.1364/ol.30.002233DOI Listing
September 2005
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